Question

which of the substances have a standard heat of formation (δh°) of zero?
o2(g) at 25.0°c and 1 atm
c2h6(g) at standard conditions
cl2(g) at 2 atm
fe at 1200°c

Explanation:

The standard heat of formation ($\Delta H_{f}^{\circ}$) is zero for elements in their standard states. The standard state of an element is its most stable form at 25°C (298 K) and 1 atm pressure.

• $O_{2}(g)$ at 25.0°C and 1 atm is the standard state of oxygen, so $\Delta H_{f}^{\circ}=0$.
• $C_{2}H_{6}(g)$ is a compound, not an element in its standard state, so $\Delta H_{f}^{\circ}

eq0$.

• $Cl_{2}(g)$ at 2 atm is not at 1 atm (standard pressure), so $\Delta H_{f}^{\circ}

eq0$.

• Fe at 1200°C is not at 25°C (standard temperature), so $\Delta H_{f}^{\circ}

eq0$.

Answer:

$O_{2}(g)$ at 25.0°C and 1 atm